1. Field of the Invention
The present invention relates to a spiral wound type membrane element used in a membrane separation device such as a low-pressure reverse osmosis membrane separation device, an ultrafiltration device or a microfiltration device, and a method for running the same and a method for washing the same.
2. Description of the Background Art
With the recent trend toward applications of membrane separation technology to water-purification technology, the membrane separation technology is being applied as pretreatment for reverse osmosis membrane separation systems used to turn salt water into fresh water, for example. While microfiltration membranes and ultrafiltration membranes which provide large permeate flow rates are mainly used for such membrane separation, reverse osmosis membranes providing large permeate flow rates at ultra-low pressures of 10 kgf/cm.sup.2 or lower are being developed these days.
As for membrane elements used for this kind of membrane separation, hollow fiber membrane elements are often used in view of membrane area per unit volume (volumetric efficiency). However, the hollow fiber membrane elements have the disadvantage that the membranes are easily broken. If the membrane is broken, raw water is mixed into permeate to lower the separating performance.
As for membrane elements providing large membrane area, there are spiral wound type membrane elements. As compared with the hollow fiber membrane elements, the spiral wound type membrane elements are more advantageous in that they can maintain high separating performance and thus provides higher reliability.
FIG. 8 is a partially cutaway perspective view of a conventional spiral wound type membrane element and FIG. 9 is an external perspective view of the conventional spiral wound type membrane element.
As shown in FIG. 8, the spiral wound type membrane element 21 includes an envelope-like membrane (a bag-like membrane) 23 formed by putting separation membranes 26 on both sides of a permeate spacer 25 and bonding them together on three sides. The opening of the envelope-like membrane 23 is attached to a water collection pipe 22 formed of a perforated hollow pipe, and it is spirally wound around the water collection pipe 22 together with a net-like raw water spacer 24.
The raw water spacer 24 is provided to form a passage through which the raw water passes between the envelope-like membrane 23. If the thickness of the raw water spacer 24 is small, the separation membranes 26 can be efficiently accommodated but they will suffer from clogging with suspended substances. Accordingly, usually, the thickness of the raw water spacer 24 is set to about 0.7 to 3.0 mm.
A spiral wound type membrane element using a corrugating type raw water spacer (a so-called corrugating spacer) is already known, which is formed in a zig-zag shape to treat raw water, e.g., river water, containing a large quantity of suspended substances.
As shown in FIG. 9, the peripheral surface of the spiral wound type membrane element 21 is covered by a sheath 27 formed of FRP (Fiber-Reinforced Plastics), a shrink tube, or the like, whose two ends are each equipped with a packing holder 28 called an anti-telescope.
FIG. 10 is a cross section showing an example of a method for running the conventional spiral wound type membrane element. As shown in FIG. 10, a pressure vessel (a pressure-resisting vessel) 30 is formed of a tubular case 31 and a pair of end plates 32a and 32b. One end plate 32a has an inlet 33 for raw water and the other plate 32b has an outlet 35 for concentrate. The other end plate 32b also has an outlet 34 for permeate in the center.
The spiral wound type membrane element 21, to which a packing 37 is attached on the peripheral surface in the vicinity of one end, is accommodated in the tubular case 31 and both of the opening ends of the tubular case 31 are sealed with the end plates 32a and 32b. One opening end of the water collection pipe 22 is engaged with the permeate outlet 34 in the end plate 32b, and an end cap 36 is attached to the other opening end thereof.
When running the spiral wound type membrane element 21, raw water 51 is introduced from the raw water inlet 33 of the pressure vessel 30 into a first liquid chamber 38. As shown in FIG. 8, the raw water 51 is supplied from one end of the spiral wound type membrane element 21. The raw water 51 flows in the axial direction along the raw water spacer 24 and is discharged as concentrate 53 from the other end of the spiral wound type membrane element 21. The raw water 51 passed through the separation membranes 26 while flowing along the raw water spacer 24 flows into the water collection pipe 22 as permeate 52 along the permeate spacer 25 and is discharged from the end of the water collection pipe 22.
The permeate 52 is taken out from the permeate outlet 34 of the pressure vessel 30 shown in FIG. 10. The concentrate 53 is taken out through the concentrate outlet 35 from a second liquid chamber 39 in the pressure vessel 30.
When the membrane element is operated, the membrane is clogged with suspended substances in the raw water, which reduces the flux of permeate. Then the clogging substances are removed by chemical washing to recover the flux of permeate, which raises the problem that the chemical washing requires troublesome work and cost. Accordingly, with a hollow fiber membrane element, for example, it is periodically cleaned by back wash reverse filtration with permeate or air to prevent clogging. However, applying back wash reverse filtration to the conventional spiral wound type membrane element 21 causes the following problems.
FIG. 11 is a partially cutaway perspective view showing back wash reverse filtration operation with the conventional spiral wound type membrane element. As shown in FIG. 11, permeate 52 is introduced from an end of the water collection pipe 22. Since the peripheral surface of the envelope-like membrane 23 wound around the water collection pipe 22 is covered with the sheath 27, the permeate guided out from the peripheral surface of the water collection pipe 22 permeates through the envelope-like membrane 23 to flow in the axial direction inside the membrane element 21 along the raw water spacer 24 and is discharged from the end of the membrane element 21. Hence, contaminants such as suspended substances causing clogging are likely to be caught by the raw water spacer 24 before discharged to the end of the membrane element 21, causing the problem that they are not sufficiently removed.
Furthermore, as shown in FIG. 10, the gap between the inner peripheral surface of the tubular case 31 of the pressure vessel 30 and the spiral wound type membrane element 21 forms a dead space S, which causes the fluid to stay (fluid stay). When the spiral wound type membrane element 21 is used in a long time, the fluid staying in the dead space deteriorates. Especially, if the fluid contains organic matter, various germs such as microorganisms propagate and decompose the organic matter to produce a bad smell, or may decompose the separation membranes, leading to reduction in reliability.
Moreover, since the raw water is supplied from one end of the spiral wound type membrane element 21 and is discharged from the other end, the conventional spiral wound type membrane element 21 requires the packing holders 28 to prevent the envelope-like membrane 23 wound around the water collection pipe 22 from being transformed into a shape like bamboo shoots. Further, pressure loss due to the raw water spacer 24 and pressure loss due to clogging produce a difference in pressure between the inflow of raw water and the outflow of concentrate, which deforms the spiral wound type membrane element 21. In order to prevent deformation, the peripheral surface of the envelope-like membrane 23 wound around the water collection pipe 22 is covered with the sheath 27 formed of FRP, a shrink tube, or the like. This increases the parts cost and production cost.
Further, it is necessary to obtain sufficient linear velocity along the membrane surface to prevent formation of cake with contaminants in the raw water, which requires sufficient flow rate of concentrate. Increasing the flow rate of concentrate lowers recovery per membrane element and requires use of a large pump to supply the raw water, which largely increases the system cost.